A holographic multiplex recording method has been proposed in which, when recording spots of interference fringes are formed in a recording layer of a holographic recording medium by projecting an object beam and a reference beam, a beam spot which is an overlapping projection region of the object beam and the reference beam is slightly shifted along the surface of the abovementioned recording layer for each projection to thereby form the recording spots superimposed with a slight shift in the X-axis direction and the Y-axis direction (see a reference, 10 May 1996/Vol. 35, No. 14/APPLIED OPTICS P2403-2417).
At this time, as shown in FIG. 9, the recording spots 1A, 1B, 1C, 1D, 1E, and 1F are formed so as to be shifted from one another in the X-axis direction in FIG. 9 by a shift amount Δ.
For example, remaining dynamic range (DR) in the recording layer in the state where the holographic recording proceeds to the recording spot 1F in FIG. 9 is nonuniform as shown in the lower half of FIG. 9. Thus, nonuniformity is caused in the intensity of formed gratings (interference fringes), resulting in distortion of a reproduction image and an increase in bit error rate.
This will be described in more detail using FIGS. 10 to 13.
FIG. 10 shows a geometrical arrangement of superimposed holograms in typical spherical shift multiplex recording. For convenience of description, the following coordinate system is defined. That is, the X-axis is defined as the line of intersection of an incident plane of a recording optical system (a plane containing the optical axes of both the reference beam and the object beam) and the surface of the recording layer, and the Y-axis is defined as the direction orthogonal to the X-axis on the plane of the abovementioned recording layer.
Due to the geometrical shape of the recorded gratings, Bragg selectivity (Bragg mismatch with respect to the amount of shift, or a moving amount at which diffraction efficiency is nearly zero when shift motion is performed by this distance from a position providing maximum diffraction efficiency) of the holograms is the highest in the X-axis direction, and is several μm in the X-axis direction and is 100 to several hundreds μm in the Y-axis direction (see the abovementioned reference).
When actual shift amounts (ΔX and ΔY in the figure) are larger than minimum shift amounts, shift multiplex recording can be achieved. However, if the actual shift amounts are too large, recording density is lowered. An example is shown in which shift multiplex recording with a shift of ΔX in the X-axis direction is performed followed by the shift multiplex recording in the Y-axis direction after completion of the multiplexing in the X-axis direction. Of course, the order of the multiplexing in the X-axis direction and the Y-axis direction may be reversed.
In either case, if a recording area is assumed to be a circle with a radius R, Np=πR2/ΔXΔY holograms on average are superimposed on each point in the recording layer. If the effective refractive index modulation degree of a recording material is assumed to be n1, and when recording is performed under the conditions in which the refractive index modulation degree per hologram satisfies Δn=n1/Np, specific scheduling is not required.
Since the scheduling for recording (which is employed for angle multiplexing or the like) is not required to be considered as described above, the spherical shift multiplex recording is a holographic recording method advantageous in high speed recording. Here, the scheduling is a technique of controlling the amount of exposure for recording according to recording history of a recording material and remaining dynamic range. In angle multiplex recording or phase code multiplex recording, since a large number of holograms are multiplexed in the same region in a recording material, the exposure amount for recording must be increased stepwise as the multiplexing process proceeds. Although the recording scheduling is not required, a contrast distribution is generated in gratings to be recorded since remaining dynamic range and photosensitivity are nonuniform in a recording area to be newly recorded, causing a problem that distortion of a reproduction image and intensity nonuniformity are generated.
FIGS. 12 and 13 show the remaining dynamic range for the case where the radius R of the recording area is normalized to 1 and the shift amounts in the X and Y axis directions are ΔX=0.01 and ΔY=0.1, respectively. Since FIGS. 12 and 13 show computational values, the entire area for |X|, |Y|≦1 is shown, but only the region inside the recording area (X2+Y2≦R2) is practically meaningful.
The relationship between the remaining dynamic range and the recording sensitivity depends on materials and the parameters of recording beam and cannot be defined in a uniform manner. However, generally, there is a tendency that the lower the remaining dynamic range, the lower the recording sensitivity. Therefore, if the dynamic range is nonuniform as shown in the graphs, nonuniformity is caused in the intensity of the formed gratings, resulting in distortion of a reproduction image and an increase in bit error rate.